FIG. 1 shows a relay node (RN) 120 and user equipments (UEs) 131 and 132 present within an area of a base station (eNodeB: eNB) 110 in a wireless communication system 100. The relay node 120 forwards data, which is received from the base station 110, to the user equipment 132 within a relay node region. And, the relay node 120 can forward data, which is received from the user equipment 132 within the relay node area, to the base station 110. Such a base station 110 may be represented as a donor base station (donor-eNB). The relay node 120 expands a fast data rate region, enhances a communication quality on a cell edge, and supports an communication offering to a building indoor or an area exceeding a base station service area. Referring to FIG. 1, there are a user equipment (hereinafter named a macro-UE or M-UE) configured to receive a direct service from the base station like the user equipment 131 and a user equipment (hereinafter named a relay-UE or R-UE) configured to receive a service from the relay node 120 like the user equipment 132.
A radio link between a base station and a relay node may be called a backhaul link. A link from a base station to a relay node may be called a backhaul downlink. A link from a relay node to a base station may be called a backhaul uplink. A radio link between a relay node and a user equipment may be called an access link. A link from a relay node to a user equipment may be called an access downlink. And, a link from a user equipment to a relay node may be called an access uplink.
Meanwhile, in MIMO (multiple input multiple output), each transmitting antenna has an independent channel. An MIMO receiving stage estimates a channel for each transmitting antenna of an MIMO transmitting stage and then receives data transmitted from the ach transmitting antenna. Channel estimation means a process for reconstructing a received signal by compensating a signal distortion caused by fading. In this case, the fading means an effect that a strength of a signal rapidly fluctuates due to multipath-time delay in a wireless communication system environment. A reference signal known to both a transmitting entity and a receiving entity is necessary for the channel estimation. The reference signal may be briefly named an RS or a pilot. Moreover, the receiving entity determines channel information on the basis of measurement of a received reference signal and then feeds back the determined channel information to the transmitting entity.
A downlink reference signal is such a pilot signal for coherent demodulation as PDSCH (Physical Downlink Shared CHannel), PCFICH (Physical Control Format Indicator CHannel), PHICH (Physical Hybrid Indicator CHannel), PDCCH (Physical Downlink Control CHannel) and the like.